:root {
  --bg: #f4f5f7;
  --card: #ffffff;
  --text: #1c1e21;
  --muted: #6b7280;
  --accent: #2563eb;
  --accent-contrast: #ffffff;
  --given: #111827;
  --input: #2563eb;
  --error: #dc2626;
  --border: #d1d5db;
  --border-strong: #374151;
  --done: #16a34a;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #111318;
    --card: #1b1e24;
    --text: #e5e7eb;
    --muted: #9ca3af;
    --accent: #3b82f6;
    --accent-contrast: #ffffff;
    --given: #f3f4f6;
    --input: #60a5fa;
    --error: #f87171;
    --border: #33363d;
    --border-strong: #6b7280;
    --done: #4ade80;
  }
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
}

.app-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 16px 20px;
  max-width: 480px;
  margin: 0 auto;
}

.app-header h1 {
  font-size: 1.4rem;
  margin: 0;
}

.icon-btn {
  background: none;
  border: none;
  color: var(--accent);
  font-size: 1rem;
  cursor: pointer;
  padding: 6px 10px;
}

main {
  max-width: 480px;
  margin: 0 auto;
  padding: 0 20px 40px;
}

.screen[hidden] { display: none; }

.tabs {
  display: flex;
  gap: 8px;
  margin-bottom: 16px;
}

.tab {
  flex: 1;
  padding: 10px 0;
  border: 1px solid var(--border);
  background: var(--card);
  color: var(--muted);
  border-radius: 10px;
  cursor: pointer;
  font-size: 0.95rem;
}

.tab.active {
  background: var(--accent);
  color: var(--accent-contrast);
  border-color: var(--accent);
}

.level-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 12px;
}

.level-btn {
  position: relative;
  aspect-ratio: 1;
  border-radius: 12px;
  border: 1px solid var(--border);
  background: var(--card);
  color: var(--text);
  font-size: 1.2rem;
  cursor: pointer;
}

.level-btn.done {
  border-color: var(--done);
}

.level-btn.done::after {
  content: "✓";
  position: absolute;
  top: 4px;
  right: 8px;
  color: var(--done);
  font-size: 0.9rem;
}

.game-info {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 12px;
  color: var(--muted);
  font-size: 0.95rem;
}

.board {
  display: grid;
  grid-template-columns: repeat(9, 1fr);
  grid-template-rows: repeat(9, 1fr);
  width: 100%;
  aspect-ratio: 1;
  background: var(--border-strong);
  gap: 1px;
  border: 2px solid var(--border-strong);
  border-radius: 6px;
  overflow: hidden;
}

.cell {
  background: var(--card);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: clamp(14px, 4vw, 22px);
  color: var(--input);
  cursor: pointer;
  user-select: none;
  position: relative;
}

.cell.given {
  color: var(--given);
  font-weight: 600;
  cursor: default;
}

.cell.error { color: var(--error); }

.cell.selected { background: color-mix(in srgb, var(--accent) 22%, var(--card)); }

.cell.peer { background: color-mix(in srgb, var(--accent) 8%, var(--card)); }

.cell:nth-child(3n) { border-right: 2px solid var(--border-strong); }
.cell:nth-child(9n) { border-right: none; }

.board .cell:nth-child(n) { border-bottom: 1px solid var(--border); }

/* thick borders every 3 rows: rows are 0-indexed groups of 9 cells */
.board .row-border-bottom { border-bottom: 2px solid var(--border-strong) !important; }

.numpad {
  display: grid;
  grid-template-columns: repeat(5, 1fr);
  gap: 8px;
  margin-top: 16px;
}

.num-btn {
  padding: 14px 0;
  border-radius: 10px;
  border: 1px solid var(--border);
  background: var(--card);
  color: var(--text);
  font-size: 1.1rem;
  cursor: pointer;
}

.num-btn.erase {
  grid-column: span 2;
  font-size: 0.85rem;
  color: var(--muted);
}

.actions {
  margin-top: 16px;
  display: flex;
  justify-content: center;
}

#restartBtn {
  padding: 10px 20px;
  border-radius: 10px;
  border: 1px solid var(--border);
  background: var(--card);
  color: var(--muted);
  cursor: pointer;
}

.overlay {
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.5);
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 20px;
}

.overlay[hidden] { display: none; }

.overlay-card {
  background: var(--card);
  color: var(--text);
  padding: 28px;
  border-radius: 16px;
  text-align: center;
  max-width: 320px;
  width: 100%;
}

.overlay-card h2 {
  margin-top: 0;
  color: var(--done);
}

.overlay-card button {
  display: block;
  width: 100%;
  margin-top: 12px;
  padding: 12px 0;
  border-radius: 10px;
  border: none;
  cursor: pointer;
  font-size: 1rem;
}

#winNextBtn {
  background: var(--accent);
  color: var(--accent-contrast);
}

#winCloseBtn {
  background: transparent;
  border: 1px solid var(--border);
  color: var(--text);
}